Direct acquisition of very large PN sequences in GPS systems

ABSTRACT

A receiver and method for acquiring a large pseudorandom (PN) sequence such as a GPS P(Y) code signal with poor or no knowledge of an external time reference are disclosed. A reference PN sequence is sectioned into a predetermined number of subsequences. The large PN sequence to be acquired is received from a remote source. The received PN sequence is correlated with each of the predetermined number of subsequences simultaneously. The large PN sequence is acquired, and a GPS solution may be provided, in the event the correlation produces a correlation between the received PN sequence and one of the predetermined number of subsequences. The large PN sequence may acquired without knowledge of an external time reference in a reasonable amount of time (e.g., less than ten minutes).

FIELD OF THE INVENTION

The present invention generally relates to the field of GPS systems andparticularly to acquisition of PN sequences in GPS systems.

BACKGROUND OF THE INVENTION

In GPS P(Y) code acquisition, the PN spread spectrum sequence is a oneweek long sequence having a rate of 10.23 MHz (6,187,104,000,000 bits).With no knowledge of an external time reference and with present digitalcorrelator technology, it is impractical to accomplish a directacquisition of a very large PN sequence such as the GPS P(Y) code in areasonable amount of time (e.g., less than ten minutes).

Typical P(Y) code acquisition techniques involve correlating a referencePN signal with the transmitted PN sequence at all possible codepositions. This correlation generally involves sequentially advancingthe reference PN sequence with respect to the transmitted PN sequenceuntil correlation is detected (indicating identity of the sequences).However, with very large length PN sequences and no knowledge of anexternal time reference, it is often impractical to sequentiallycorrelate at all code positions in a reasonable amount of time due tothe finite number of correlation processors available. Thus, there liesa need to provide a system and method for acquiring a relatively large,finite PN sequence in a reasonable period of time without knowledge orwith poor knowledge of an external time reference.

SUMMARY OF THE INVENTION

The present invention is directed to a method for acquiring apseudorandom (PN) sequence. In one embodiment, the method includes stepsfor sectioning a reference PN sequence into a predetermined number ofsubsequences, (with possible gaps between the subsequences) receivingthe PN sequence from a remote source, correlating the received PNsequence with each of the predetermined number of subsequencessimultaneously, and in the event the correlating step producescorrelation between the received PN sequence and one of thepredetermined number of subsequences, acquiring the received PNsequence.

The present invention is further directed to a GPS receiver foracquiring a GPS P(Y) code signal without knowledge or with poorknowledge of an external time reference. In one embodiment, the receiverincludes a first filter and amplifier for selecting a predeterminedfrequency band of a received GPS signal and for amplifying the filteredsignal, a first converter for converting the received GPS signal into anintermediate frequency signal, a second filter and amplifier forfiltering and amplifying the intermediate frequency signal, a secondconverter for converting the filtered and amplified intermediatefrequency signal into a digital signal representative of the GPS signal,and a processor for receiving the digital signal, the processor beingconfigured to simultaneously correlate the digital signal with aplurality of subsequences of a reference signal such that the GPS signalis acquired upon correlation of the digital signal with one of theplurality of subsequences of the reference signal.

It is an object of the present invention to provide a method andappartus for acquiring large PN sequences in a short period of time.

It is a feature of the present invention to acquire a large PN sequencein six minutes or less.

It is a feature of the present invention to acquire a large PN sequenceby holding the reference PN sequence fixed and waiting until thereceived PN sequence generates a match between the sequences.

It is an advantage of the present invention to acquire a large PNsequence without a time reference.

It is an advantage of the present invention to acquire a large PNsequence with a poor time reference.

It is to be understood that both the foregoing general description andthe following detailed description are exemplary and explanatory onlyand are not restrictive of the invention as claimed.

The accompanying drawings, which are incorporated in and constitute apart of the specification, illustrate an embodiment of the invention andtogether with the general description, serve to explain the principlesof the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The numerous advantages of the present invention may be betterunderstood by those skilled in the art by reference to the accompanyingfigures in which:

FIG. 1 is a block diagram of a GPS receiver and processor operable totangibly embody the present invention;

FIG. 2 is a block diagram illustrating the preprocessing of a referencePN sequence in accordance with the present invention;

FIG. 3 is a block diagram of a correlator operable with the presentinvention; and

FIG. 4 is a flow diagram of a method for acquiring a PN sequence inaccordance with the present invention.

DETAILED DESCRIPTION OF THE INVENTION

Reference will now be made in detail to the presently preferredembodiment of the invention, an example of which is illustrated in theaccompanying drawings.

Referring now to FIG. 1, a block diagram of a GPS receiver operable totangibly embody the present invention will be discussed. The GPSreceiver 100 includes an antenna 110 for receiving a pseudorandom (PN)sequence signal such as the P(Y) code transmitted from a space vehiclein a global positioning system (GPS). The received signal is applied tothe input of a preselect filter 112 (e.g., a band pass filter) forselecting the frequency of the received signal. The filtered signal isamplified by an amplifier 114 that is preferably a low noise amplifier(LNA). The signal is then converted from a radio-frequency (RF) signalto an intermediate frequency (IF) signal by RF to IF converter 116. Theintermediate frequency signal is then further filtered by band passfilter 118 and further amplified by amplifier 120. The signal is thendown converted into in-phase (I) and quadrature (Q) components by I, Qdown converter 122. The I and Q components are converted into a digitalsignal by A/D converter 126 and then provided to GPS processor 128. GPSprocessor 128 preferably comprises or includes a signal processor(digital, optical, analog, etc.) for implementing faster processing ofsignal processing algorithms (e.g., correlations, convolutions, etc.). Afrequency synthesizer 124 provides a reference signal of a predeterminedfrequency to RF to IF converter 116, I, Q down converter 122, and to A/Dconverter 126. A real-time clock 130, random-access memory (RAM) 132 andnon-volatile memory (NVM) 134 couple with GPS processor 138 via bus 136.GPS processor may receive an auxiliary signal 138 from an auxiliarysignal source, for example as a reference signal for providing a Dopplercorrection of a received GPS signal. The result of the acquisition ofthe PN signal, such as a coarse/acquisition (C/A) solution of a GPSsignal may be provided at an output 140 of GPS processor 128 accordingto known GPS processing techniques.

Referring now to FIG. 2, the preprocessing of a reference PN sequence inaccordance with the present invention will be discussed. The referencePN sequence 210, the sequence against which an incoming PN sequencereceived by receiver 100 is correlated, is sectioned into a plurality ofsubsequences: subsequence 1 (212), subsequence 2 (214), up tosubsequence N (224) where N is the number of subsequences. Gaps canexist between the subsequences as shown in FIG. 2 with gap 1 (232)through gap M (244) where M is the number of gaps. The subsequences andgaps can be of differing lengths. In the particular case of GPS P(Y)code acquisition, the PN spread spectrum sequence is a one-week longsequence having a rate of 10.23 MHz (6,187,104,000,000 bits). The entirereference P(Y) code is sectioned into many (N) spaced fixed reference PNsubsequences 212-224 with many (M) gaps 232-244. The GPS P(Y) code canbe segmented into approximately 15000 subsequences of 204,600 bits (20milliseconds) each, which comprises only a small portion of the entiresequence. Since the phase of the incoming signal is correlated with anin-phase and quadrature reference, 7500 in-phase and quadraturesubsequence pairs are used in a preferred embodiment of the presentinvention. In this case N=7500. Different numbers of subsequences andnumbers of bits in the subsequences can also be used. The reference PNsequence, either as a whole sequence 210 or as individual subsequences212-224, may be stored in non-volatile memory 134 until required byprocessor 128 (e.g., to perform a correlation with an incoming PNsequence). The reference sequence may be stored in memory in a digitizedformat.

Referring now to FIG. 3, a block diagram of a correlator operable withthe present invention will be discussed. The acquisition of an incomingPN sequence 310 involves near continuous correlation of the fixedreference PN subsequences 212-224 simultaneously with a received,incoming PN sequence 310. The subsequences used with a typicalcorrelator 312 are arranged in groups such that group 1 includes I and Qsubsequence 1 (212) followed by I and Q subsequence 2 (214), I and Qsubsequence 3 (216) and I and Q subsequence 4 (218). Subsequence 2 (214)is selected to be one bit later than subsequence 1 (212) in thereference PN sequence. I and Q subsequence 3 (216) is selected to be onebit later than subsequence 2 (214) and I and Q subsequence 4 (218) isone bit later than subsequence 3 (216). The groups of four I and Qsubsequences are evenly spaced over the incoming PN sequence 310. Withcorrelator response time of approximately 396 nanoseconds or four bitsof the incoming PN sequence 310, the four subsequences (212-218) areoverlaid with their one bit delay with respect to each other. Acorrelation is made between the 204,600 bits of incoming PN sequence 310and the 15,000 sequences 212 through 224 simultaneously. The overlay offour bits for each I and Q subsequence in a group allows for thecorrelation of these four I and Q subsequences during a correlatorresponse period resulting in 1875 subsequence groups to be correlatedsimultaneously. The correlator 312 can consist of 15,000 parallelcorrelation channels or 15,000 individual correlators to perform thecorrelation operation simultaneously between the incoming PN code andthe 1875 subsequence groups in the same fashion as described for thefirst group of I and Q subsequences and the corresponding correlator 312channel. This correlation operation is performed in a near continuousfashion on the incoming PN sequence limited by the correlator responsetime of 396 nanoseconds in this example. The output of the I and Qcorrelations can be combined mathematically in an envelope or powerdetector.

With 1875 subsequence groups evenly spaced over the entire incoming PNsequence 310 of one week or 604,800 seconds, one subsequence group ispresent in each 320 second period of the code or one uniform gap length.In the current embodiment with a correlator response time of 396nanoseconds or four PN code bits, the incoming PN code shifts four bitsrelative to the 1875 subsequence groups while a correlator operation isperformed. To search the entire window of uncertainty, approximately 320seconds would be required. In this example, the worst case acquisitiontime of a desired correlation is approximately 320 seconds or under sixminutes. Other correlation intervals can be used depending on thecorrelator and other numbers of subsequences can be used resulting indifferent acquisition times.

Instead of sequentially advancing reference PN sequence 210 with respectto incoming PN sequence 310, the incoming PN sequence propagates throughthe receiver 100 and at times determined by the correlator responsetime, the subsequences 212-224 are correlated with the incoming sequence310. As one of the reference PN subsequences 212-224 and incoming PNsequence 310 align mathematically, correlation is detected at thecorrelation point and provided at the output 314 of correlator 312. Thismeans the receiver doesn't need a predetermined time reference but justneeds to wait for a correlation match. Correlator 312 may be optimizedfor correlating a large, fixed sequence such as incoming PN sequence310. Correlator 312 may be implemented as a hardware embodiment as acomponent of GPS processor 128 or as a program of instructions oralgorithm executed by a digital signal processor or microprocessorembodied as processor 128 or preferably may be implemented as an opticalcorrelation embodied as processor 128.

Referring now to FIG. 4, a method for acquiring a PN sequence inaccordance with the present invention will be discussed. The method 400includes a step 410 for sectioning a reference PN sequence 210 into aplurality of subsequences 212-224. An incoming PN sequence 310 to beacquired is received by receiver 100 at step 412. Incoming PN sequence310 continuously propagates through the receiver 100 and shifts withrespect to reference subsequences 416 such that incoming PN subsequence310 is correlated with all reference subsequences simultaneously at step416. A determination is made at step 418 whether correlator 312 detectscorrelation between incoming PN sequence 310 and one of the referencesubsequences 212-224. Incoming PN sequence 310 continues to shift withrespect to reference subsequences 212-224 and in the event correlationis not detected, the correlation step 416 continues at a different pointin the incoming PN subsequence 310. In the event correlation is detectedat the correlation point step 418, incoming PN sequence is acquired atstep 420 such that a GPS solution may be provided by processor 128.

Although the invention has been described with a certain degree ofparticularity, it should be recognized that elements thereof may bealtered by persons skilled in the art without departing from the spiritand scope of the invention. One of the embodiments of the invention canbe implemented as sets of instructions resident in a memory 132 or 134of one or more processors such as processor 128 generally as describedin FIG. 1. Such a processor may be considered generally as part of acomputer system. Until required by the computer system or processor, theset of instructions may be stored in a computer readable memory.Further, the set of instructions can be stored in the memory of anothercomputer and transmitted over a local area network or a wide areanetwork, such as the Internet, as desired. Additionally, theinstructions may be transmitted over a network in the form of an applet(a program executed from within another application) or a servlet (anapplet executed by a server) that is interpreted or compiled aftertransmission to the computer system rather than prior to transmission.One skilled in the art would appreciate that the physical storage of thesets of instructions or applets physically changes the medium upon whichit is stored electrically, magnetically, chemically, physically,optically or holographically so that the medium carries computerreadable information.

It is believed that the direct acquisition of very large PN sequences inGPS systems of the present invention and many of its attendantadvantages will be understood by the foregoing description, and it willbe apparent that various changes may be made in the form, constructionand arrangement of the components thereof without departing from thescope and spirit of the invention or without sacrificing all of itsmaterial advantages. The form herein before described being merely anexplanatory embodiment thereof. It is the intention of the followingclaims to encompass and include such changes.

What is claimed is:
 1. A method of acquiring a pseudorandom (PN)sequence, comprising: sectioning a reference PN sequence into apredetermined number of subsequences; receiving the PN sequence from aremote source; propagating the PN sequence through a correlator;correlating the received PN sequence with each of the predeterminednumber of subsequences simultaneously as the received PN sequencepropagates through the correlator; waiting a period of time until thepropagating received PN sequence correlates with one of thepredetermined number of subsequences at a correlation point in thereceived PN sequence; and acquiring the PN sequence at the correlationpoint.
 2. A method as claimed in claim 1, said predetermined number ofsubsequences being a plurality of uniformly spaced apart subsequences ofthe reference PN sequence with uniform gap lengths between the spacedapart subsequences.
 3. A method as claimed in claim 2, wherein theperiod of time is a uniform gap length or less to reach the correlationpoint.
 4. A method as claimed in claim 1 wherein the PN sequence is aGPS P(Y) code signal.
 5. A program of instructions storable on acomputer readable medium and executable by a computer for executingsteps for a acquiring a pseudorandom (PN) sequence, the stepscomprising: sectioning a reference PN sequence into a predeterminednumber of subsequences; receiving the PN sequence from a remote source;propagating the PN sequence through a correlator; correlating thereceived PN sequence with each of the predetermined number ofsubsequences simultaneously as the received PN sequence propagatesthrough the correlator; waiting a period of time until the propagatingreceived PN sequence correlates with one of the predetermined number ofsubsequences at a correlation point in the received PN sequence; andacquiring the PN sequence at the correlation point.
 6. A program ofinstructions as claimed in claim 5, said predetermined number ofsubsequences being a plurality of uniformly spaced apart subsequences ofthe reference PN sequence with uniform gap lengths between the spacedapart subsequences.
 7. A program of instructions as claimed in claim 5,wherein the period of time is a uniform gap length or less to reach thecorrelation point.
 8. A program of instructions as claimed in claim 5,said PN sequence being a GPS P(Y) code signal.